AAUSat-2 (also spelled as AAUSAT-II) is a student-developed picosatellite (CubeSat) of Aalborg University, Aalborg, Denmark. The overall mission goal is education - by participation and involvement in all aspects of spacecraft design and development. The satellite is a joint venture of five university institutes. The project started in the summer 2003. The satellite caries two science experiments, an ADCS system and a gamma ray detector. 1)

Background: AAUSat-1 (also known as AAU CubeSat) of Aalborg University was launched on June 30, 2003 as a secondary payload in a multiple CubeSat launch on a Rockot vehicle from Plesetsk, Russia. Contact with the CubeSat was established. AAUSat-1 was was alive for two and a half months before the battery had lost too much capacity to continue operations. During this time only a limited amount of data was successfully downlinked from the satellite due to an undisclosed problem on the satellite transmitter resulting in very weak signals being transmitted. - Still, as a first step into picosatellite technology and student built satellites, the project was considered a success.

AAUSat-2 conforms to the CubeSat standard in size (cube of 10 cm side length) and in mass (≤ 1 kg). Its structure uses an aluminum frame and CFRP (Carbon Fiber Reinforced Plastic) side panels. The ADCS (Attitude Determination and Control Subsystem) provides initial stabilization of the CubeSat after deployment to enable fine-pointing and attitude tracking for scientific experiments. Actuation is provided by 3 magnetorquers and 3 momentum wheels; attitude sensing uses a 3-axis magnetometer, 6 rate-gyro chips, and 6 sun sensors with photodiodes. 2)

Figure 3: Illustration of the ADCS (image credit: Aalborg University)

The EPS (Electric Power Subsystem) with surface-mounted solar panels (2 panels on each side) is providing xy W of average power.

The OBC (Onboard Computer) employs an ARM-7 microprocessor with built-in CAN (Controller Area Network) controller (Atmel: AT91SAM7A1). It interfaces with the other subsystems via the CAN bus.

RF communications: The radio link is a half-duplex at 437.425 MHz (UHF) implementation with a bandwidth of 6 kHz. FFSK (Fast Frequency Shift Keying) modulation is used. The data rates (1200, 2400, or 4800 baud) can be set by a command from the MCC (Mission Control Center). The default data rate is 1200 baud.

Launch: A launch of AAUSat-2 as a secondary payload took place on April 28, 2008 on a PSLV launch vehicle of ISRO from SDSC (Satish Dhawan Space Centre), Sriharikota, India. The primary payload on this multi-satellite flight was CartoSat-2A.

The launch of secondary payloads is being shared by IMS-1 (Indian Microsatellite-1) of ISRO with the following CubeSats or nanosatellites: CanX-2 of UTIAS/SFL, Toronto, Canada, AAUSat-2 of Aalborg University, Denmark; COMPASS-1 of the University of Applied Science, Aachen, Germany; Delfi-C3 of the Technical University of Delft, The Netherlands; SEEDS-2 of Nihon University, Japan; CUTE-1.7+APD-2 CubeSat of the Tokyo Institute of Technology; NTS (Nanosatellite Tracking of Ships) of Com Dev / UTIAS/SFL, Toronto, Canada; and Rubin-8-AIS an experimental space technology mission of OHB-System, Bremen, Germany.

The launch of the 8 nanosatellite payloads was executed under a commercial contract between the University of Toronto, COSMOS International (a company of the OHB Fuchs Gruppe, Bremen, Germany), and the Antrix Corporation of Bangalore, India (the latter is the commercial arm of ISRO).

Six custom-built XPOD (eXperimental Push Out Deployer) units were provided by UTIAS/SFL (Toronto, Canada) to deploy all smallsats - except the CUTE-1.7+APD-2 (3.5 kg) double cube of TITech which used its own TSD deployer system.

Orbit: Sun-synchronous near-circular orbit, altitude = 615 km x 634 km, inclination = 97.9º, the period is 97.4 minutes, the local time on the descending node (LTDN) is at 9:30 hours.

Mission status:

• In 2012, the AAUSat-2 mission was officially retired by the project — but the CubeSat is still up and running. As of March 2012, the project receives beacons regularly. Beacons from the sunny side and from the eclipse phase of the orbit. Beacons in "LOS" of the eclipse show 8.2 V on the battery and 8.3-8,4 V in "daytime" which means the batteries are still reasonably ok. - The spinning of the CubeSat stabilized around 2 Hz or a little bit more (Ref. 4).

• In April 2011, AAUSAT-2 is still alive sending beacons every 30 seconds (after 3 years in orbit). The spacecraft is still rebooting on a regular basis. The project cannot find a definite reason for this behavior - but suspects that relatively high temperatures are prevailing on the Arm/ main OBC (~ 60ºC) causing a skew timing error on the CAN bus (Ref. 4).

- So far, there were more than 14,000 power cycles since the launch of the CubeSat. Since last year, the rate has increased by ~ 20%. The power cycling resets all subsystems including the ADCS.

Figure 6: Illustration of the power reset cycles of AAUSat-2 on March 24, 2011 (image credit: AAU) 3)

- The project suspects a current in the solar cell wires providing a very small momentum like a water mill wheel. In any case AAUSAT-2 is slowly spinning up. In April 2011 it rotates with approximately 2.5 Hz.

- Despite the tumbling the project is still able to communicate with AAUSAT-2. However, due to the tumbling and minor blind spots of the antenna, there are quite a few bit errors in the data from the satellite. - The battery voltage is 8.2-8.4 V after more than 1600 orbits (Ref. 4).

• The spacecraft is still alive in late April 2010 but with a number of drawbacks (on April 28, 2010 the mission was on orbit for two years). The project receives beacons on a daily basis. 4)

- The spacecraft is rebooting 10-14 times on a daily basis. The project reasons the rebooting is due to a mis-timing on the Can bus which in turn may be due to the fact that the ARM-7 onboard is around 60ºC. After every reboot the flight plan is empty and therefor no detumbling/ADCS is running.

- The tumbling rate is above 2 Hz (stabilization problems). The project reasons that the solar cells and the wiring are responsible for the increased tumbling.

• Contact with the spacecraft was established shortly after deployment.

Sensor complement:

Experimental gamma ray detector, developed and provided by the Danish National Space Center (DNSC). The instrument is based on a solid CdZnTe (Cadmium Zinc Telluride) crystal. The size of the crystal is 10 mm x 10 mm x 4 mm with a mass of < 5 gram. The specific crystal has a detection range from 5 to 300 keV with a resolution of 3 keV at 60 keV, making this small crystal a very usable scientific device.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.